This is usually the result of the vars.txt file not getting written correctly. Check that you have a vars.txt file in the main Subscriber directory, as well as in each list directory, and that the file is not blank.

Blank pages can also be caused if you created ANY lists that have invalid characters in it. For example, if you try to add a plug in it will not work, or if you use the form generator you will get a blank page if you created any lists at all (even if it is not the one that you are using) that have invalid characters in the list name or ID.

Delete any lists that you are not using, if it is the one that you are using delete it and re-create it.